Sulfuric acid, in particular concentrated sulfuric acid, typically contains small amounts of various nitrogen oxides, collectively referred to as NOx. Nitrosyl sulfuric acid is believed to be the predominate NOx species, but other oxides of nitrogen may also be present. In some applications, the presence of even small amounts of nitrosyl sulfuric acid or other NOx can be problematic. For example, a specification of less than 5 parts per million (ppm) NOx for electrolyte grade sulfuric acid has been set the United States General Services Administration (Federal Specification O-S-801F, Notice 2, 27 Jul. 2011). Sulfuric acid from a typical production line including those that use spent sulfuric acid as a feed material must be post-treated to reduce the NOx below that 5 ppm level.
U.S. Pat. No. 3,012,854 discloses treatment of sulfuric acid with hydrazine sulfate or dihydrazine sulfate to eliminate oxidizing compounds and cause the acid to pass the Murray test.
U.S. Pat. No. 5,955,050 discloses a process for removal of NOx from sulfuric acid comprising treating the sulfuric acid with hydrazine, sulfamic acid or urea.
Treatments to remove NOx, such as those referenced above, are done as a separate post-treatment of the sulfuric acid. It would be advantageous to have a process for NOx removal which is integral with the sulfuric acid production.